This invention relates to a method of manufacturing printed wiring boards with a multiplicity of small plated through-holes or via holes.
In typical prior art, a printed wiring board is manufactured by the steps of: preparing a substrate comprising an insulating core plate and copper plate films formed on both the surfaces thereof, drilling the substrate to form a plurality of through-holes, electroless-plating with copper to form thin copper films on the walls of the through-holes, subjecting the substrate obtained to a conventional photolithography process to form wiring patterns of copper on the core plate, and electro-plating with copper the substrate obtained to form thick wiring patterns of copper.
Recently, the wiring densities of printed wiring boards have remarkably become high with the reduction in the diameters of via-holes and in the widths of wiring patterns.
However, with via-holes of 0.4 mm or less in diameter, bubbles, produced by the reaction of a plating solution or by the immersion of the substrate into a plating bath, adhere on the surfaces of the substrate and the walls of the through-holes, and disturb the good control in the thickness of a plates film. It is observed that most bubbles on the surface of a base substrate do not easily move their locations during a copper plating process.
In order to overcome such troubles, it has been tried to vibrate the substrate in a plating bath, or to agitate a plating solution. However, it has not succeeded in completely removing bubbles, particularly those on inner walls of via-holes.
In general, it is desirable that a copper film for wiring is at least 30 microns in thickness. However, as shown at portions A and B in FIGS. 1 and 2, it has been difficult to avoid the facts that due to the presence of bubbles the copper film 2 on a substrate 1 is partially thinned to a thickness of only 5 microns or partially cut away on the surface and/or on the inner walls of via-holes 3.
Therefore, when the printed wiring board is subjected to a heat shock test or to a soldering process for mounting electrical parts on the printed wiring board, the thickness variation in the plated film sometimes caused a burnout of the thinned portion of the plated film on the board.